1. Field
Exemplary embodiments of the present invention relate to a method of fabricating a gallium nitride (GaN) based semiconductor device, and more particularly, to a method of fabricating a GaN based light emitting diode with a vertical structure using a GaN substrate as a growth substrate.
2. Discussion of the Background
In general, since Group-III element nitrides such as gallium nitride (GaN) and aluminum nitride (AlN) have excellent thermal stability and a direct-transition-type energy band structure, the Group-III element nitrides have recently come into the spotlight as materials to be used for light emitting devices in the visible and ultraviolet light wavelength emission regions. Particularly, blue and green wavelength light emitting devices using indium gallium nitride (InGaN) may be used in various applications, such as large-sized full-color flat panel displays, traffic lights, indoor illumination, high-density light sources, high-resolution output systems, optical communications, and the like.
Since it may be difficult to fabricate a homogeneous substrate on which a Group-III nitride semiconductor layer can be grown, the Group-III nitride semiconductor layer has been grown on a heterogeneous substrate having a similar crystalline structure through a process such as metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or the like. A sapphire substrate having a hexagonal system structure may be used as the heterogeneous substrate. Recently, a technique for fabricating a high-efficiency light emitting diode (LED) with a vertical structure has been developed by growing epitaxial layers, such as nitride semiconductor layers on a heterogeneous substrate, such as a sapphire substrate, bonding a support substrate to the epitaxial layers, and then separating the heterogeneous substrate using a laser lift-off technique or the like. Since the heterogeneous substrate such as the sapphire substrate and the epitaxial layers grown thereon may have different physical properties, the growth substrate can be easily separated by using the interface between the heterogeneous substrate and the epitaxial layer. The separated heterogeneous substrate may be reused as a growth substrate or for another purpose through subsequent surface treatment thereof.
However, the epitaxial layer grown on the heterogeneous substrate may have a relatively high dislocation density due to a lattice mismatch and a difference in thermal expansion coefficient between the epitaxial layer and the growth substrate. It has been found that the epitaxial layer grown on the sapphire substrate generally has a dislocation density of 1×108/cm2. There may be a limitation on improvement of the light emitting efficiency of an LED using an epitaxial layer having such a high dislocation density.
In order to reduce a crystal defect resulting from the growth of the epitaxial layer on the heterogeneous substrate, there has been a recent attempt to use a GaN substrate as a growth substrate. The GaN substrate is a homogeneous substrate with respect to a GaN based semiconductor layer grown thereon, so that the crystal defect may be greatly reduced to enable growth of a high-quality GaN based semiconductor layer.
However, since the GaN substrate is a homogeneous substrate with respect to the GaN based semiconductor layer grown thereon, it may be difficult to separate the growth substrate from an epitaxial layer by using the interface between the substrate and the epitaxial layer. Accordingly, removing the GaN substrate by means of grinding after the growth of the epitaxial layer is completed may be considered. In this case, however, the GaN substrate cannot be reused, which may increase a total production cost.
Alternatively, separating a GaN substrate from an epitaxial layer by applying a laser lift-off technique may be considered. However, since the GaN substrate is a homogeneous substrate with respect to the epitaxial layer grown thereon, the wavelength range of a laser that may be selected for the laser lift-off technique may be considerably narrow. Accordingly, upon irradiation of the selected laser, most of the laser may be lost due to absorption in the GaN substrate while passing through the GaN substrate.